US2754475A

US2754475A - Borehole diameter and lateral depth of fluid invasion indicator

Info

Publication number: US2754475A
Application number: US73375A
Authority: US
Inventors: Russell G Norelius
Original assignee: Lane Wells Co
Current assignee: Lane Wells Co
Priority date: 1949-01-28
Filing date: 1949-01-28
Publication date: 1956-07-10
Anticipated expiration: 1973-07-10

Description

July 10, 1956 R. G. NORELIUS BoREHoLE DIAMETER AND LATERAL DEPTH DE FLUID INvAsIoN INDIE-:TDR

4 Sheets-Sheet 1 Filed Jan. 28, 1949 I' D ERD/HL DISTHNCE' I'PdM XIS INVENTOR. @USSELL @./QEL/US BY HTTENE'V July 10, 1956 R. G. NoRELlUs 2,754,475

BOREHOLE DIAMETER AND LATERAL DEPTH OF FLUID INVASION INDICATOR Filed Jan. 28, 1949 4 Sheets-Sheet 2 o 7 L Jgdf'a.

IN VEN TOR. @USSEL 6I /PEL /06' July 10, 1956 R. G. NoRELlus BOREHOLE DIAMETER AND LATERAL. DEPTH OF FLUID INVASION INDICTOR 4 Sheets-Sheet 3 Filed Jan. 28, 1949 /fldO lll ya f Dwmwmrf :Ems r9. y G m 6 l m f m m LIJ m 8 w 5 R mm M se [j sla G4 m m 41| e L w m www 7 r m. wf v m m IIB INVENTOR. RUSSELL G. /OPZ/l/S BY M July 1o, 1956 R Filed Jan. 28, 1949 4 Sheets-Sheet 4 INVENTOR. /QUSSELL G. 0262/05 United States Patent BOREHOLE DIAMETER AND LATERAL DEPTH OF FLUID INVASION INDICATOR Russell G. Norelius, Huntington Park, Calif., assignor to Lane-Wells Company, Los Angeles, Calif., a corporation of California Application January 28, 1949, Serial No. 73,375

6 Claims. (Cl. 324-1) This invention relates, in general, to electrical logging of well boreholes and more particularly to methods and apparatus for determining the borehole diameter and the depth of invasion of fluid into the penetrated formations.

This invention utilizes the principals of but constitutes an improvement over the conventional multiple electrode method of so-called deep penetration resistivity, electrical coring or electrical logging of well boreholes, an early disclosure of the basic principles of which may be found in the patent to Schlumberger 1,819,923. In this so-called deep penetration resistivity logging, as heretofore practiced, an electric current is conducted through a conductor in a multi-conductor cable to a current input electrode located within and in electrical contact with the fluid of the borehole. The return path for the current from the input electrode extends through the surrounding earth formations to another remotely located current input electrode suitably located either within the uid in the well borehole or elsewhere in the ground at the earth surface, and the direction of llow f the current is substantially radially diverging in all directions for a substantial distance in all directions from the input electrode through the surrounding fluid in the borehole and out through the surrounding formations. As a result of the beforementioned radially diverging current flow paths in all directions from the input electrode, an innite number of concentric spherical equipotential surfaces may be considered as surrounding the input electrode as a center. Potential differences existing between any two of these so-called concentric spherical surfaces may be tested by means of a pair of potential pick-up electrodes spaced apart radially with respect to one another and with respect to said input electrode. Resistivity measurements are thus generally made by means of a separate pair of longitudinally spaced potential pickup electrodes usually positioned on the longitudinal axis of the well borehole either above or below the beforementioned current input electrode, the distance of the axial spacing between the pick-up electrodes and the current input electrode being equal to the desired, socalled lateral depth of penetration of the resistivity measurement desired to be made.

Resistivity measurements thus made actually are measurements of the average resistivity of the bodies of formation and well borehole fluid contained within the spherical shell having a thickness equal to the spacing of the pickup electrodes and surrounding the current input electrode as a center. Due to differences in the electrical characteristics of the well borehole fluid and various portions ot' the formation surrounding the well borehole, intersected by the spherical potential measurement shell, the said shell may actually be somewhat distorted in shape and consequently the resulting resistivity measurementY may be only approximately indicative of the average resistivities of the various materials included within the beforementioned spherical shell. Thus, by this method it has been, heretofore, impossible to determine the resistivity of the surrouding formations with suicient quan- Fice titative accuracy and, furthermore, it has been impossible to obtain accurate information as to the lateral depth of any discontinuities in the electrical characteristics of the surrounding formations.

This invention is an improvement over the hereinbefore described electrical logging system, which makes available an electrical log of the well borehole from which the electrical resistivities of the formations surrounding the well borehole and the lateral depth of electrical discontinuities therein may be more accurately determined and, furthermore, this invention provides a means for obtaining an electrical log of the well borehole diameter.

Accordingly, an object of this invention is to provide an electrical system for determining the diameter of the well borehole.

Another object of this invention is to provide an electrical method and apparatus for making a continuous log of borehole diameters correlated with depth.

Still another object of this invention is to provide an electrical method and apparatus for determining the lateral depth of penetration of well borehole fluid into the penetrated formations surrounding the borehole, and to make a continuous log thereof correlated with borehole depth.

Another object of this invention is to provide an electrical method and apparatus for providing an improved electrical log of a well borehole wherewith more and better information can be obtained relative to the physical and electrical characteristics of the pentrated formations surrounding the well borehole.

The objects of this invention are accomplished, in general, by establishing an electrical field converging substantialy radially through surrounding formations upon an input electrode in the fluid within a well borehole, loeating a pair of probes or pick-up electrodes in electrical contact with the fluid in the well borehole, said pick-up electrodes of said pair being spaced apart axially with respect to one another a given fixed distance and the said pair being spaced axially with respect to the beforementioned input electrode, and moving or reciprocatingsaid pair of pick-up electrodes axially in said borehole with respect to said input electrode to effect a continuous change of axial spacing between said pair of electrodes and said input electrode and observing or recording the resultant simultaneously changing relationships between the potential picked up between said pair of electrodes and the said axial spacing between said pair of electrodes and said input electrode. Resistivity measurements are thus, in effect, made at continuously varying lateral depths in the fluid in the borehole and formation surrounding the borehole, whereby the positions of any lateral discontinuities in electrical resistivity become apparent.

These and other objects, features of novelty and advantages will be evident hereinafter.

In the drawings which illustrate preferred embodiments and modes of operation of the invention and in which like reference characters designate the same or similar parts throughout the several views:

Figure 1 is a vertical elevational View of the general assembly of the electrode assembly portion of the apparatus of the invention as it would appear when suspended in a well borehole in readiness for operation.

Figures 2a and 2b are portions of a schematic diagram of the apparatus of the invention included in that shown in Figure l.

Figure 3 is an enlarged view of the graphical record produced by the indicating or recording portion of the apparatus shown in Figure 2:1.

Figure 4 is an elevational view of the general assembly of an alternative form of that portion of the apparatus of the invention illustrated in Figure 1 as it would appear u when suspended in a well borehole in readiness for operation.

Figures Sa and b are portions of a schematic diagram of the apparatus of the invention included in that shown in Figure 4.

Figure 6 is an enlarged graphical record produced by the indicating or recording portion of the apparatus shown in Figure 5a.

Figure 7 is an enlarged detail view of Figure 5b.

Figure 8 is a schematic diagram of an alternative arrangement of that portion of the apparatus shown in Figure 5a.

Figure 9 is a graphical illustration of typical forms of curves which may be indicated or produced by the apparatus of Figure l and Figure 4 under different conditions.

The apparatus is as follows:

Referring first primarly to Figure l, 10 is an electrode assembly adapted to be lowered into a well borehole on a conductor cable 11. The cable 11 may be reeled in or paid out as required from a suitable hoist drum 12 located at the top of the well. The conductor cable 11 contains a plurality of insulated conductors which interconnect the electrode assembly 10 with the electrical apparatus at the top of the well as will be hereinafter more fully described.

The electrode assembly 10 comprises an elongated tubular body 13 having a cable socket 14 at the upper end thereof into which the conductor cable 11 is anchored and through which the insulated conductors pass from the cable end to the apparatus contained within the electrode body 13. At the lower end of the tubular body 13 is a tubular section 15 of enlarged diameter which carries at an intermediate portion thereof a cage spring assembly as shown at 16 which is composed of at least three outwardly bowed leaf springs, as shown at 17, adapted to press outward in sliding frictional engagement with the inside surface of the well borehole in a manner well known in the art, to effect a centering of the lower end of the electrode assembly within the well borehole. The lower extremity of section is rounded or pointed, as shown at 19, to facilitate its passage through the borehole, and it carries adjacent the upper end thereof a pair of longitudinally spaced annular electrodes P1 and P2 hereinafter referred to as the potential pickup electrodes.

A sleeve member 23 having a cage assembly 24 fixed adjacent the upper end thereof and of a construction similar to that of the hereinbefore mentioned cage 16, is carried upon the tubular body member 13 and is free to slide longitudinally thereon between the shoulder 26 formed at the lower end of the cable head 14 and the shoulder 27 formed at the juncture of the tubular body portion 13 with the enlarged section 15 of the electrode assembly. An annular electrode member C1, hereinafter referred to as the current input electrode, is carried on the sleeve member 23 adjacent its lower end 28.

The entire exterior surface of the electrode assembly 10, including the tubular member 13 and the sleeve member 23, but with the exception of the centering cage springs at 16 and 24, the cable head 14 and the outer cylindrical surfaces of the annular electrodes P1, P2 and C1, is covered with a uid tight layer of insulating material of rubber, neoprene or the like substance as is well known in the electrical logging art, to minmize any undesirable effects which the other electrically conductive portions of the electrode supporting structure might otherwise have upon the operation of the electrode system.

Reference is now directed primarly to Figures 2a and 2b, in which the electrical circuits and apparatus housed within the electrode assembly 10 and those provided at the earth surface at the top of the well, interconnected by means of the conductors within the conductor cable 11 are diagrammatically illustrated. The current input electrode C1 is connected through a variable resistance 30 contained within the said electrode assembly 10 and through insulated conductor 31, within the conductor cabi@ 11 all@ through surface conductor lead 32 to one terminal 33 of a suitable current supply 34, preferably an alternating current supply. The other terminal 35 of the current supply 34 is grounded at C2 through conductor 36. If desired, the ground C2 may be effected by connecting the conductor 36 to the metal sheath of the conductor cable 11 as is frequent and well known practice in the electrical logging art. The pickup electrodes P1 and P2 are connected through

insulated conductors

38 and 39 contained Within the conductor cable 11 and thence through

surface conductors

40 and 41, respectively, to the

terminals

42 and 43 of a suitable voltmeter or galvanometer device 45. The

cable conductors

31, 38 and 39 make connection with slip rings rotatably mounted upon the conductor cable drum 12, and electrical connection is completed between these slip rings and the input terminal 33 of the current supply 34 and the

beforementioned surface conductors

32, 40 and 41 through stationary brushes as illustrated respectively at 46, 47 and 48 as is conventional practice with such apparatus. v

The voltmeter 45 includes a movable arm 49 having a pen or other suitable marking device at its outer end adapted to rest upon a movable chart or strip of graph paper 50 for the purpose of making a permanent graphical record as illustrated at 51 and which will be more fully described hereinafter in connection with the operation. The chart 50 is movable between a pair of rollers or reels 52 and 53 preferably at a rate which is proportional to the rate of motion of the cable 11 into or out of the well borehole. Such proportional motion may be accomplished by coupling an idler pulley over which the cable passes, as shown at 54, with the graph paper transporting mechanism through suitable means such as by a flexible shaft, belt or the like mechanical device similar to those disclosed in the patents to Jakosky Re. 21,797 or Elliott 2,222,608 or by electro-mechanical means as disclosed in Bowsky et al. 2,142,555 or as illustrated in Figures l and 2a hereof in which a Selsyn generator 55 is driven by the idler pulley 54 and this generator is in turn electrically coupled through suitable conductors as conventionally illustrated at 56 to a Selsyn motor 57 which in turn remotely drives the paper transporting roll 52 through suitable reduction gearing as shown at 58.

Figure 3 shows a view of the chart 50, sufficiently enlarged and positioned in such a manner as to illustrate the correlation of the graph 51 with the axis of the well borehole and the surrounding formation, as schematically illustrated in Figure 2b.

As beforementioned, the sleeve member 23 and the tubular body member 13 are longitudinally slidable a limited distance with respect to one another. The tubular body 13 of the electrode assembly 10 thus may be lowered through the sleeve member 23 from the position illustrated in Figure 1 while the sleeve is suspended within the borehole by the friction of cage 24 against the borehole wall, to increase the spacing between the input electrode C1 and the pair of pickup electrodes P1 and P2 as diagrammatically indicated in Figure 2b. Thus the pickup electrodes P1 and P2 may be moved downward relative to the input electrode C1 to their extreme lower positions illustrated at dotted lines P1" and P2 at which position the electrodes have their maximum longitudinal spacing or the pickup electrodes may be moved to any intermediate position such as that illustrated in dotted lines at P1 and P2.

Circular lines

59 and 60 drawn about the current input electrode C1 as a center, illustrate the position of the spherical boundaries of the zone or pherical shell across which the potential measurement is taken when the potential pickup electrodes are located as shown at P1 and P2. The circular lines 61 and 62 drawn about the current input electrode C1 as a center illustrate the spherical boundary surfaces of the zone or spherical shell across which the potential measurement is taken when the potential pickup @.ltrodes are located in an intermediate position as indicated in dotted lines at P1 and Pz. Similarly, circular lines 63 and 64 also drawn about the current input electrode C1 as a center illustrate the position of the spherical boundary surfaces of the zone or spherical shell across which the potential measurement is taken when the potential pickup electrodes are located at their lowerrnost positions as illustrated in dotted lines at P1 and P2.

Referring now primarily to Figures 4, a, 5b, 6 and 7, in which the alternative or modified version of the apparatus of Figure 1 is illustrated, 70 is an electrode assembly as it would appear suspended in a well borehole 71 upon a conductor cable 72 which extends to a cable drum as illustrated at 73. The electrode assembly 70 includes an elongated, tubular body portion 75 having adjacent its upper and lower ends, cage spring assemblies as shown at 76 and 77. The

cage assemblies

76 and 77 are of more or less conventional construction and each have at least three outwardly bowing springs as shown at 78 and 79 adapted to bear outwardly in resilient sliding engagement with the inside surface of the well borehole wall to maintain the electrode assembly 70 centered upon the axis of the borehole.

The tubular body portion 75 of the electrode assembly is covered from end to end with a fiuid tight layer of insulating material in the same manner as hereinbefore mentioned in connection with the apparatus of Figure 1. A plurality of longitudinally spaced, annular shaped potential pickup electrodes as shown at P1 to P23, are carried by and suitably attached to the insulating covering of the tubular body portion 75 in such manner as to be insulated from one another and from the metal portions of the electrode assembly but exposed to electrical contact with surrounding borehole fluid. An annular shaped, current input electrode, as shown at C1 is similarly positioned on the insulating covering on the tubular body member 75 adjacent the uppermost electrode P1 of the potential pickup electrodes. All of the electrodes C1 and P1 to P23 inclusive, are thus electrically insulated from each other and from the metal portions of the electrode assembly 70 but exposed to electrical contact with surrounding borehole uid.

The apparatus contained within the electrode assembly 70 and that located at the surface of the ground outside of the well borehole and the electrical cable connections therebetween, are diagrammatically illustrated in Figures 5a, 5b and 7. The current input electrode C1 is connected within the tubular body member 75 through connection 80 to the lower end of the cable conductor 81 and thence through stationary brush 82 at the cable drum 73 and through surface conductor 84 to one terminal 85 of the current supply 86. The other terminal of the current supply 86 is grounded as shown at C2 through ground conductor 87. Each of the series of annular potential pickup electrodes P1 to P23 inclusive, are respectively connected within the tubular body portion 75 of the electrode assembly through the resistors shown at R1 to R23 and conductors 1 to 23, each to a separate segment of a commutator device 90. The commutator device 90 is provided, as here illustrated, with twenty-four commutator segments. Twenty-three of the commutator segments are connected to the twenty-three potential pickup electrodes leaving one commutator segment, that shown at 91, electrically isolated and not connected to any conductor.

A pair of circumferentially spaced brushes 92 and 93 are adapted to be rotated in sliding contact with the commutator segments by means of a drive shaft 94 which is coupled to the shaft of a motor 95, all of which is housed within the tubular body 7S of the electrode assembly 70. A pair of slip rings 96 and 97 are coaxially mounted upon the shaft 94 and connected electrically with the beforementioned brushes 92 and 93, respectively. A pair of

stationary brushes

98 and 99 make sliding electrical contact with the slip rings 96 and 97 and these brushes are, in turn, electrically connected through

connections

100 and 101 to the bottom end of the

cable conductors

102 and 103 within the tubular body 75, which are, in turn, connected at the top end of the conductor cable through

brushes

104 and 105 at the cable drum 73 with

surface conductors

106 and 107 which lead to

input terminals

108 and 109 of amplifier 110. The

output terminals

111 and 112 of the amplifier 110 are connected through

conductors

113 and 114 to a pair of

vertical deflection plates

115 and 116 of acathode ray oscilloscope as illustrated at 118. The amplifier may include in its output, suitable rectifier and filter circuits, not shown, but more fully described hereinafter.

Suitable current is supplied for operation of the motor 95 within the electrode assembly 70 from a current supply 120 exterior to the well borehole through the cable conductor 121 by way of hoist drum brush 122 and return through the ground connections.

The

brushes

92 and 93 of the commutator device are circumferentially spaced a distance equal to the circumferential center to center spacing of adjacent commutator segments so that a pair of adjacent commutator segments are always connected across the

cable conductors

102 and 103, and as the brushes of the commutator device are rotated, the adjacent pairs of commutator segments are connected to the

conductors

100 and 101 in rapid succession.

A horizontal sweep trigger circuit of conventional design is provided for the cathode ray oscilloscope as indicated at 123. The

control input terminals

124 and 125 of the horizontal sweep trigger circuit are connected respectively through

conductors

126 and 127 to the

beforementioned conductors

106 and 107. The

output terminals

129 and 130 of the horizontal sweep trigger circuit are connected through

conductors

131 and 132 to the

horizontal deflection plates

133 and 134 respectively of the cathode ray oscilloscope 118.

The cathode ray oscilloscope 118 may be of conventional design having an electron gun portion as diagrammatically illustrated at 136 and a fluorescent screen at 137. v

The cathode current supply and cathode, anode and grid high voltage supply for theV electron gun 136 may be of conventional design and therefore are omitted from the drawings.

A typical wave form is illustrated at 138 as it would appear on the uorescent screen of the oscilloscope 118 under the operating conditions illustrated in Figures 5a and 5b as will be more fully explained hereinafter in connection with the operation.

Figure 6 shows the beforementioned wave form 138 suiciently enlarged and positioned in such a manner as to illustrate the correlation of the curve with respect to the axis 140 of the well borehole and the surrounding formations as schematically illustrated in Figure 5b. A wave of similar form is also shown in Figure 9b which will be described hereinafter.

Referring now to Figure 8, a further modification of the recording portion of the apparatus adapted to be used in connection with the apparatus illustrated in Figures 5a and 5b is illustrated. The purpose of this apparatus is to make a continuous running record of the borehole diameter and depth of well fiuid invasion, correlated with depth of the Well borehole. In this latter case, the

connections

131 and 132 leading from the horizontal sweep trigger circuit are connected to the

horizontal deflection plates

134 and 135 of the oscilloscope 137 in the same manner as that hereinbefore described in connection with Figure 5a. However, the

leads

113, 114 from the amplifier 110, instead of being connected to the

vertical deflection plates

115 and 116, as shown in Figure 5a, are connected to the electron gun 136 of the oscilloscope, the connection 114 being connected to the cathode, and connection 113 being connected to the control grid at 140. In this latter arrangement, no vertical deflection plates need be used.

All of the other connections leading from the apparatus 7 of Figure b to the commutator motor current supply 120, the horizontal sweep trigger circuit 123, amplifier 110 and input electrode current supply 86, are the same as those shown and hereinbefore described in connection with Figure 5a.

A suitable lens system is provided as shown at 141, adapted to focus any image formed upon the fluorescent screen 137 of the oscilloscope 118 upon a photographic strip of paper or film 142.

The photographic film or strip 142 is adapted to be transported longitudinally between

rollers

144 and 145 by means of a suitable coupling of the same type and construction as that shown and hereinbefore described in connection with the apparatus of Figures 1 and 2a whereby the said longitudinal motion of the film or strip 142 is directly proportional to and correlated with the motion and depth of the electrode assembly 70 within the well borehole.

A variable density, photographic image of the uorescent line 175 is formed at 146 on the strip 142, by means ot' the lens system 141 which, in conjunction with the motion of the strip 142, produces a variable density record as will be described more fully in connection with the operation.

The operation of the invention is as follows:

Referring first to Figure l, the electrode assembly 10 is lowered into the well borehole by paying out conductor cable 11 from the conductor cable drum 12. When the electrode assembly 10 has reached a point in the well borehole where it is desired to make measurements, the lowering operation is temporarily halted. Following the aforesaid initial downward motion of the electrode system 10 within the well borehole, the conductor cable 11 may be hoisted a short distance sufficient to raise the tubular body portion 13 and the elements associated therewith including the pair of potential pickup electrodes P1 and P2, upward with respect to the sleeve member 23 during which movement the sleeve member 23 will be suspended stationary within the well borehole by the frictional engagement of the cage 24 with the borehole wall. The potential pickup electrodes P1 and P2 may then have assumed a position with respect to the current input electrode C1 approximately that shown in Figure 1 and in solid lines in Figure 2b. At this position, while a current, preferably an alternating current, is applied t0 the penetrated formations between the ground C2 and the current input electrode C1 within the fluid in the well borehole, the potential picked up between the potential pickup electrodes P1 and P2 will be approximately that due to the effective resistivity of the volume of earth formation and well borehole Huid contained within the spherical shell bounded by the two concentric spherical surfaces represented in cross-section by the

circular lines

59 and 60, in which the potential pickup electrodes P1 and P2 lie.

By lowering away on the conductor cable 11, the tubular body portion 13 of the electrode system may be lowered through the sleeve member 23 while the sleeve member 23 remains stationary within the well borehole supported by the 'cage 24. The potential pickup electrodes P1 and P2 may thus be moved relative to the current input electrode C1 to any lower position desired within the beforementioned limits of such motion, two of such positions being illustrated in dotted lines at P1 and P2 and at P1 and P2". At these latter positions, as in the initially described position, the potential there picked up between the potential pickup electrodes, for example, in the position shown at P1 and P2', will be that corresponding to the resistivity of the volume of formation and well uid contained within the spherical shell formed between the imaginary concentric spherical surfaces represented in cross-section by the circular lines at 61 and 62. In the lowermost position shown for the electrodes P1 and P2", the potential picked up between the potential pick electrodes will be that corresponding to the resistivity of the volume of formation and the well fluid contained within the spherical shell formed between the concentric spherical surfaces represented in cross-section by the circular lines 63 and 64. As beforementioned, the minimum and maximum spacing between the pair of potential pickup electrodes P1 and P2 and the current input electrode C1 is determined by the distance through which the tubular body portion 13 may be moved longitudinally within the sleeve member 23, the minimum spacing being determined by contact between the shoulder 27 of tubular section 15 and the lower end 28 of the tubular member 23, and the maximum spacing being determined by the contact between the shoulder 26 at the bottom end of the cable socket 14 with the upper end 32 of the said sleeve member 23. Greater latitude of motion may obviously be provided than that herein illustrated. As the pair of potential pickup electrodes P1 and P2 are moved downward from their most adjacent position with respect to the current input electrode C1, the spherical shell across which the potential is being measured by the potential pickup electrodes P1 and P2 may be considered to expand continuously until it reaches a condition of maximum possible diameter approximating that illustrated by the curved lines 63 and 64 which occurs at the maximum downward displacement of the tubular body portion 13 with respect to the sleeve portion 23 of the electrode assembly 10.

In the condition where the spacing between the current input electrode C1 and the potential pickup electrodes P1 and P2 is a minimum as when

shoulders

27 and 28 are in contact with one another, the spherical shell across which the potential is being measured by the potential pickup electrodes P1 and P2 is represented by dotted lines at and 56. In this position is is apparent that all of the volume of the

spherical shell

55, 56 is located within the fluid within the borehole. As the

spherical shell

55, 56 is caused to expand wholly within the well uid by downward motion of the pair of pickup electrodes P1 and P2, with respect to the current input electrode C1, the well uid being of substantially homogenous resistivity, the potential picked up by the potential pickup electrodes P1 and P2 will remain substantially constant as graphically illustrated at on the charts 50 of Figures 2a and 3. As the outer portion of the

spherical shell

55, 56 passes into or intersects the wall of the well borehole from a position in which it was wholly within the well fluid to a position in which a portion of it extends into the surrounding formation at point 65, the average resistivity of the material contained within the volume of the spherical zone undergoes an abrupt change which, in a case where the fluid-invaded formation at that point has a resultant resistivity substantially higher than the borehole fluid alone, results in a relatively sharp, upward bend of the curve as shown at 151 in Figures 2a and 3. As the imaginary spherical resistivity measurement shell continues to expand by further separation of the pair of pickup electrodes P1 and Pz downward from the current input electrode C1, the inner spherical surface of the portion of the expanding shell radially opposite electrode C1 finally passes outward beyond the adjacent borehole wall into the formation as shown at 59, 60 at which point the rate of change or rate of increase of average resistivity of the material within the volume of the spherical shell decreases, resulting in a leveling off or reduction in slope of the resistivity curve as shown at 152 in Figures 2a and 3. Thereafter as the spherical measurement shell continues to expand, the rate of change of resistivity within the spherical shell increases more slowly as the portion of the volume of the spherical shell contained wholly within the surrounding formations increases in proportion to those portions shown at 155 and 155 which remain contained within the well borehole uid. This latter part of the curve is shown at 156 in Figures 2a and 3. When the outer spherical surface boundary of the spherical shell as shown at 61, 62 reaches the outer limit of the well-fluid invasion zone, as represented by the irregular boundary line 157, and begins to enter the uninvaded formation therebeyond which may be of higher resistivity by reason of the absence thereof any conductive well fluid infiltrate, the average resistivity of said volume of formation contained within the spherical shell 61, 62 undergoes another abrupt change which in this case is a further increase in the rate of incrase of resistivity as shown by the abrupt upward bend of the resistivity curve at 158, in Figures 2a and 3. This upward bend continues until the inner boundary of the spherical shell 61, 62 passes out into the formation beyond the interface 157 after which the curve again undergoes a gradual reduction in slope as illustrated in that part of the curve shown at 160.

By placing the chart, as shown in Figure 3, with the lefthand vertical zero axis on the center line or axis of the well borehole, it is then possible to determine by the shape of the curve that the radius of the well borehole is equal to the distance O-R and that the distance of well iiuid invasion into the formation is equal to O-l. The maximum lateral depth of measurement as represented by the maximum diameter of the spherical shell 63, 64 is equal to the distance O-D.

If the physical arrangement of the electrode system permitted the distance between the pair of pickup electrodes P1 and P2 and the current input electrode C1 to be continued to be increased without limit, the spherical shell 63, 64 would correspondingly expand without limit as a result of which the portion of the curve 160 would be projected laterally a coresponding further distance in the manner illustrated in dotted lines at 180 in Figure 8b along which it would approach asymptotically the resistivity value of the undisturbed formation extending laterally from the well borehole beyond the fluid invaded zone. By thus projecting the curve 160 in the manner illustrated in dotted lines at 180, an indication of the approximate resistivity of the undisturbed and univaded formation may be obtained, as more fully explained hereinafter in connection with Figure 8.

For the purpose of simplicity of illustration, the electrodes C1, P1 and P2 have been illustrated as having relatively wide longitudinal spacing therebetween as compared to the borehole diameter. In actual construction where higher definition and greater accuracy in the determination of a well borehole diameter and iluid invasion distance is required, the potential pickup electrodes P1 and P2 would be spaced closer together and the apparatus would be constructed such that the current input electrode C1 and the pair of potential pickup electrodes P1 and P2 could be brought into closer proximity to one another than is indicated by the scale of the drawing, whereby the spherical shell illustrated in dotted lines at 55, 56 would be radially thinner and would have an initial diameter considerably smaller than that illustrated in Figure 1. With such closer electrode spacing arrangement, as the pickup electrodes are moved downward with respect to the current input electrode, the spherical shell would undergo a substantial expansion prior to moving of its external spherical boundary surface into contact with the inside surface of the well borehole. With such reduced thickness, of the spherical shell, the break in the curve at the point shown at 151 in Figure 3, would be more abrupt. The same would be true of the break in the curve shown at point 158. Thus, in the actual construction, greater denition in the curve shown in Figure 3 would be obtained.

During measurements as hereinbefore described, the potential pickup elecrtodes P1 and Pz may be moved up and down repeatedly with respect to the current input electrode C1 by alternately hoisting and lowering the conductor cable 11 by means of the drum 12. Since the chart 50 is coupled to the drum 12 as hereinbefore described, it will move back and forth a proportional distance between the rollers 51 and 52 permitting the pen arm 49 of the meter 45 to retrace the curve repeatedly.

When it is desired to take measurements of the well borehole diameter and distance of fluid invasion at arother point within the well borehole, the whole electrode assembly 10 may be raised or lowered to such point and the hereinbefore described process repeated.

The current electrode C1 is preferably connected to the conductor cable 55 through a variable resistance 30 housed suitably within the tubular member 13 of the electrode assembly 10 and so coupled, one element thereof to the body 13 and another element thereof to the sleeve member 23, as to vary the resistance in accordance with a function of the separation of the pickup electrodes P1 and P2 from the current electrode C1. In general, the resistance 30 should be so coupled and so constructed as to decrease in value as the electrode separation is increased, at such a rate as to compensate for the volume increase of the before-mentioned spherical shell accompanying such separation of the electrodes. The rate at which the resistance'thus changes preferably should be such that if the electrode system were placed within a surrounding material which is homogeneous with regard to its resistivity, separation of the potential pickup electrodes from the current input electrode in the manner hereinbefore described would result in a straight horizontal line on the chart of Figure 3 extending from R to D. Then, when the electrodes are immersed in a nonhomogeneous surrounding material, such as the surrounding borehole fluid and adjacent formations illustrated in Figure 2b, the vertical deflections of the curve 156 of Figure 3 will represent changes in measured resistivity which are solely due to lateral variations and discontinuities in the electrical resistivity of those surrounding materials.

The rate of change of the resistance 30 to fulfill the beforedescribed condition can be expressed approximately as follows:

b d a R da where Ro is the sum of all the series resistances in the potential pickup circuit including the resistance 30, the resistances of the cable conductors 3S and 39, the

surface conductors

40 and 41 and the input resistance of the meter 45, and where a and b are the distances of separation between the electrodes C1 and P1 and between C1 and P2, respectively. Resistance 30 should thus change in value and at a rate necessary to presume this relationship.

Referring now to the modified version of the apparatus 0f this invention as illustrated in Figures 4, 5a, 5b, 6 and 7, the operation is as follows:

The electrode system 70 is first lowered into the well borehole through the well fluid therein to a point opposite the formations to be logged. During the following logging operations, the motor is set into operation by supplying it with current from the current supply 120. Continuous rotation is thereby imparted from the motor 95 through the shaft 94 to the commutator device including the

brushes

92 and 93 and the slip rings 94 and 95. As the

brushes

92 and 93 pass over the commutator segments, the two adjacent commutator segments over which the

brushes

92 and 93 pass, are momentarily connected through the slip rings 94 and 95, brushes 96 and 97 to the

conductors

100 and 101 and from there the circuit is completed through the

cable conductors

102 and 103 and

surface connecting wires

106 and 107 to the

input terminals

108 and 109 of the amplier 110. For example, at the particular instant of operation illustrated in Figure 5b, the

brushes

92 and 93 rest upon and make electrical contact with the commutator segments to which

conductors

14 and 15 lead. These

conductors

14 and 15 each make connection through resistors R14 and R15 to the potential pickup electrodes P14 and P15, respectively. Therefore, at this instant the potential being picked up by the potential pickup electrodes P14 and P15 is that appearing across the spherical shell between the

spherical surfaces

170 and 171. The potential thus picked up from electrodes P14 and P15 are applied, as before described, to the amplifier 110. The resultant amplified current output from amplifier 110 is applied through

conductors

113 and 114 to the

vertical defiection plates

115 and 116 of the oscilloscope 118, with the resultant defiection of the electron beam within the oscilloscope to impinge upon the fluorescent screen 137 at the point 172 at the particular instant of operation illustrated. Assuming the commutator device is rotating in a clockwise direction as indicated by the arrow, the

brushes

92 and 93 will next be moved into contact with the commutator segments to which

conductors

15 and 16, resistors R15 and R15, and potential pickup electrodes P15 and P16 are attached. The potential picked up between the potential pickup electrodes P15 and P15 will next be applied to the circuit leading to amplifier 110. Thus, each adjacent pair of commutator segments and the corresponding potential pickup electrodes will be connected in rapid succession to the measuring circuit leading to the amplifier 110, the output from which is applied to the

vertical deflection plates

115 and 116 of the oscilloscope 118. Therefore, for each rotation of the commutator device the potential picked up between each pair of pickup potential electrodes from electrode P1 to electrode P23, is measured in rapid succession causing, in effect, the earth formation adjacent the electrode assembly within the well borehole to be, in effect, scanned laterally by a stepwise expanding, spherical shell of potential-difference measurement which expands from its minimum diameter, as illustrated at 169, 170 to its maximum diameter illustrated at 172, 173, once for each rotation of the commutator device.

It is to be noted that twenty-four commutator segments are shown in the commutator device. Twenty-three of these commutator segments are connected to the potential pickup electrodes P1 to P23. The twenty-fourth cornmutator segment which is indicated at 91 is electrically isolated, without any connection thereto. As the

brushes

92, 93 pass over commutator segment 91 there is a momentary open circuit in the circuit leading to the amplifier 110 and also to the horizontal sweep trigger circuit 123. This open circuit interval is utilized by suitable means well known in the art to actuate the horizontal sweep trigger circuit to produce a saw toothed pulse for each revolution of the commutator device. The potential thus generated by the horizontal sweep trigger circuit 123 thus triggered in synchronism with the rotation of the commutator device, is impressed upon the

horizontal deflection plates

133 and 134 of the oscilloscope 118 to produce one full scale sweep of the electron beam horizontally across the oscilloscope screen for each revolution of the commutator device. A curve or wave form is thus formed on the fluorescent material 137 of the oscilloscope of the type illustrated at 138, the vertical deection of which is proportional to a function of or representative of the resistivity measurements between the successively connected pairs of potential pickup electrodes and the horizontal deflection of which is preferably proportional to or capable of calibration in terms of the lateral distance of the successive spherical shells of potential measurement from the axis of the well borehole.

When alternating current is supplied by the current supply 86 and utilized in the electrical logging operations, the amplifier 110 preferably includes suitable rectification and filtering means to produce an output at 111 and 112 which when impressed upon the

deflection plates

115 and 116 of the oscilloscope will result in a relatively smooth, continuous curve instead of individual pulses for each commutator connection.

Figure 6 and Figure 9B illustrate the curve 138 as produced on the fluorescent screen of the oscilloscope, enlarged sufiiciently in each case to be correlated in scale with the axis of the well borehole and the adjacent formation. As hereinbefore described in connection with Figures l, 2a, 2b and 3, the breaks in slope appearing in the curve 138 at 176 and 172 are indicative of the positions of the well borehole wall and the lateral limit of well fiuid invasion into the adjacent formation. Thus it is apparent that at a distance R from the well borehole axis, as shown in Figures 5a and 6, the spherical shell of resistivity measurement first contacts the inside surface of the well borehole. Similarly, at a distance I from the axis of the well borehole, the upward break in the curve, appearing at 172, indicates the lateral distance of invasion of well borehole fluid filtrate into the formation.

As hereinbefore described in connection with Figure l to Figure 3, the plurality of electrodes including the current input electrode C1, and the potential pickup electrodes P1 to P23, have for convenience been illustrated as being relatively widely separated and relatively few in number. Actually, in the practical construction of the apparatus, a larger number of potential pickup electrodes more closely spaced are employed. For example, in order to measure with greater definition and accuracy the well borehole diameter and the depth of well fiuid invasion of a depth of, for example, three feet laterally from the axis of the well borehole 144 potential pickup electrodes longitudinally spaced approximately 1A; inch from center to center may be employed and the minimum spacing between the current input electrode C1 and the first adjacent potential pickup electrode P1 may be in the order of 1/2 of an inch. Each electrode should be relatively thin in order to avoid the establishment of any substantial, longitudinal, short-circuiting current path therethrough from electrode to electrode; for example, an electrode thickness of approximately 1/16 of an inch in thickness has been found to be satisfactory and the outside diameter of the exposed surface of these electrodes may be in the order of three inches.

The resistances R1 and R23, inclusive, may be graduated in value in accordance with the relationship hereinbefore mentioned in connection with the apparatus of Figures l and 2b to compensate for the different diameters of the spherical potential measurement shells measured by the pairs of potential pickup electrodes. For example, the total resistance, R0 present in the measuring circuit connected through the commutator brushes 92 and 93 including the resistances R14 and Ris may be expressed as:

where a equals the distance from electrode C1 to electrode P14, and where b equals the distance from electrode C1 to electrode P15, and K is a suitable constant depending upon the current input, average resistivity of the formations to be measured and the full scale range of the type of potential measuring device at the well surface such as the oscilloscope 118 and amplifier 110.

It may, under some conditions, be desirable to introduce compensation for the varying electrode spacing by other means, particularly where the potential picked up by the pickup electrodes is introduced into an extremely high impedance input, voltage measuring circuit such as may be present in a vacuum tube voltmeter circuit such as that which may comprise amplifier 110. In such latter case, a suitable electrical control current or p-ulse may be communicated from the commutator motor current supply 120 through conductor 178 to the amplifier and return through suitable ground connections to effect a periodic variation in the gain of the amplifier. Proper synchronism is assured, for example, by employing an alternating current generator for the current supply and a synchronous motor at 95 for driving the commutator. A synchronously driven gain control may be employed in the amplifier 110, driven by a portion of the alternating current from the said current supply 120, by means of which the required vari- Ro--K ations in gain will be synchronized with the comrnutator device 90.

Referring now to Figure 8, the rapidly, stepwise varying potential differences obtained from the plurality of potential pickup electrodes instead of being applied to the vertical deflection plates of an oscillograph after amplification as shown in Figure a, are here applied between the cathode and control grid of the oscilloscope 118. Thus, the amplied, picked-up potentials are applied from the amplifier 110 through

conductors

113 and 114 to the control grid 140 and to the cathode 136, respectively, of the oscilloscope 118. The quantity of electrons in the electron beam impinging upon the fluorescent screen 137 is thus varied or modulated in accordance with the different or varying potentials picked up by the potential pickup electrodes P1 to P23, respectively. The electron beam is deflected horizontally in the same manner as hereinbefore described in connection with Figure 5a by means of the sweep potential pulses obtained from the trigger circuit 123 and applied to the

horizontal deflection plates

134 and 135 through

conductors

131 and 132, respectively. In this manner, for each complete cycle of potential measurements from potential pickup electrodes P1 to P23 occurring during each rotation of the commutator device, the electron beam is deflected horizontally to form a horizontal fluorescent line, as shown at 175 on the fluorescent screen 137. The intensity or brilliance of the lluorescence of line 175 is governed by the potential applied to the control grid 140 as hereinbefore mentioned. The image of the uorescent line 175 is projected by means of a suitable lens 141 onto a light sensitive strip of material such as a photographic lm 142 which is moved longitudinally across the rollers 144 and 145 a distance proportional to the motion of the electrode assembly 70 within the well borehole. A variable density recording is thus produced on the photographic strip 142 having the appearance as illustrated at 146 with variations in density appearing where the curves illustrated in Figure 3 and Figure 6, exhibited breaks in slope. By inspection of the variable density record thus made, it is possible to determine that the borehole diameter corresponds to the horizontal position of the first visible abrupt change in density as illustrated at 177 and that the outer limit of well fluid invasion is at a distance from the borehole axis as indicated by the horizontal position of the next visible abrupt change in density as illustrated at 17S. By correlation between the electrode spacing of the electrode assembly 70 within the well borehole and the magnitude and rate of the horizontal sweep of the electron beam in the oscilloscope 113 as manifested by the horizontal length of the variable density line upon the photographic strip 142, the actual dimensions of the well borehole and distance of lateral invasion of well borehole iluid may be approximately established.

By the method and apparatus illustrated in connection with Figure 8, a continuous graphical record may be made of well borehole diameter and depth of lluid invasion, correlated with well borehole depth.

The resistivity curves shown in Figures 3 and 6 are of the general character which result where the Well borehole iluid has a relatively low resistivity, for example, three ohms per square meter per meter, the uninvaded formation a relatively high resistivity, for example, sixty ohms per square meter per meter, and Where the intermediate, invaded portion of the formation has an intermediate resistivity, for example, thirty ohms per square meter per meter. The relative resistivities of these lexplored zones varies considerably from formation to formation resulting in measured curves of different form and character but Wherever a lateral discontinuity in resistivity is present, the curve will indicate it by an abrupt change in slope.

Referring now to Figure 9, the curves A, B and C are representative of the type of curves obtainable by the process and apparatus of Figures 1 to 6, inclusive, under different conditions. For curve A, the well borehole fluid, the surrounding invaded formation and the formation beyond the zone of invasion are assumed to have resistivities of 3, 30 and 3 ohm-meters per square meter, respectively. Under these conditions, as the pickup electrodes are moved from their initial position close to the input electrode outward to their maximum spacing, the initial horizontal portion of the curve shown at 200 is indicative of the resistivity of the Well borehole fluid. The upward break or abrupt increase in slope of the curve shown at 201 is indicative of the electrode spacing at which the spherical potential measurement shell begins to enter the adjacent well borehole wall. The following portion of the curve shown at 202 of gradually reducing slope is representative of that portion of the resistivity measurement between the expanding spherical potential measurement shell which includes a portion of the well borehole Huid and the surrounding lluid invaded formation. The abrupt reversal of slope of the curve shown at 203 is indicative of the electrode spacing and the corresponding lateral depth of the resistivity measurement shell as it begins to pass from the invaded formation zone into the surrounding uninvaded formation as illustrated by the position of the circles 61 and 62 or

circles

170 and 171 in Figures 2b and 5b, respectively. The balance of the curve illustrated at 204 has a downward slope reverse to that shown at 202 by reason of the fact that the surrounding uninvaded formation has a relatively low resistivity as compared to that of the invaded zone. From curve A, it may be concluded that the formation surrounding the well borehole is probably a dense formation of low porosity containing a connate fluid of low resistivity such as salt water. Some such condition must exist, otherwise the invaded formation would not display a resistivity which is so high relative to the resistivities of the mud and of the uninvaded formation lying beyond the zone of invasion.

Now if the portion of the curve shown at 202 is projected forward on the same trend and is illustrated by the dotted lines shown at 205, it is apparent that the curve is approximately asymptotic to the 30 ohm ordinate of the graph. From this it may reasonably be concluded that if the lateral penetration of the resistivity measurement is extended to such an extent that substantially the entire area or volume of the resistivity measurement shell lies Within a surrounding fluid invaded formation of unlimited extent, the curve would approach an ordinate value of 30 ohms as a limit. Thus, it appears that the invaded formation zone has a resistivity of approximately 30 ohm-meters per square meter as beforestated.

Similarly, if the portion of the curve shown at 204 is projected forward as illustrated by the dotted lines at 206, it apparently approaches asymptotically a resistivity value of 3 ohm-meters per square meter as a minimum limit. From this it may be concluded that the true resistivity of the uninvaded formation lying beyond the zone of invasion is approximately 3 ohm-meters per square meter as beforestated.

Referring now to the curve B, as in the previous curve, the horizontal portion shown at 208 represents the resistivity of the well borehole fluid. The abrupt change in slope of the curve shown at 209 represents the lateral distance at which the resistivity measurement spherical shell enters the surrounding borehole wall. The following upwardly sloping curve shown at 210 represents the effect upon the measured resistivity of the resistivity measurement shell as more and more of its volume enters the surrounding zone of invasion of the formation. The abrupt upward increase in slope shown at 211 is indicative of the lateral distance at which the spherical potential measurement shell commences to leave the invaded zone and enters the outlying uninvaded portions of the surrounding formation.

As in the case of curve A if the portion of the curve shown at 210 of B is projected forward, as illustrated in dotted lines at 212, it apparently approaches the ohm ordinate of the graph asymptotically as a limit. From this it may be reasonably concluded that the true resistivity of the fluid invaded zone is approximately 30 ohm-meters per square meter.

The following portion of the curve shown at 213 is representative of the effect of the entrance of the spherical potential measurement shell into the surrounding uninvaded formation. If this curve is projected forward as shown in dotted lines at 214, the resultant curve apparently approaches asymptotically the 60 ohm ordinate of the graph as a limit. From this it may be reasonably concluded that the true resistivity of the surrounding uninvaded formation is approximately 60 ohm-meters per square meter.

In curve C the initial horizontal portion shown at 216 represents the resistivity of the well borehole fiuid. The abrupt upward increase in slope of the curve shown at 217 is indicative of the point at which the spherical resistivity measurement shell enters the surrounding borehole wall. The following upwardly sloping curve shown at 218 represents the increase in measured resistivity as the spherical potential measurement shell enters the surrounding fluid invaded zone. The abrupt reversal and downward slope of the curve shown at 219 indicates the lateral point at which the resistivity measurement spherical shell passes out of the invaded formation into the surrounding uninvaded formation. The downwardly sloping part of the curve shown at 220 indicated the gradual reduction in measured resistivity resulting from more and more of the spherical potential measurement shell entering the surrounding, uninvaded formation.

If, as hereinbefore described, that portion of the curve shown at 218 is projected forward sufciently far, as illustrated in dotted lines at 211, it is apparent that it approaches the 90 ohm ordinate of the graph asymptotically as a limit. From this it may be reasonably concluded that the true resistivity of the invaded zone of the formation is approximately 90 ohm meters per square meter. Likewise, if the latter portion of the curve shown at 220 is projected forward sufficiently far as illustrated in dotted lines at 222, it apparently approaches the 60 ohm ordinate of the graph asymptotically as a limit. Likewise, from this it may be reasonably concluded that the true resistivity of the surrounding uninvaded formation is 60 ohm meters per square meter as beforestated.

In the before described curves of Figure 9, the dash-dot line at 225 indicates the position of the center line or longitudinal axis of the well borehole, relative to the abscissa of these curves. The distance O--R with proper correlation of scale is indicative of the well borehole radius. The distance O-I is similarly indicative of the depth of invasion of the well borehole fluid into the surrounding formation. The distance O-D is similarly indicative of the maximum possible separation of the measuring electrodes in the well borehole and is thus the maximum lateral depth to which resistivity measurements can be made by the particular apparatus illustrated. The distance O-D and the corresponding lateral depth of possible resistivity measurements may obviously be modified and increased to any desired value within the practical limits of suitable design of the borehole electrode assembly and associated apparatus.

From the foregoing it is believed obvious that a particular advantage and point of novelty of this invention resides in the discovery that by recording the apparent resistivities of the borehole fluid and formation surrounding the borehole continuously from a point close to the axis of the well borehole to a laterally remote point in the surrounding formations, information may be obtained which is indicative of the locations of lateral discontinuities in the electrical resistivities of the surrounding materials, whereby the well borehole diameter and the depth of fluid invasion may be determined readily and with substantially greater accuracy than was ever possible heretofore. Furthermore, by this information, it is possible to determine with reasonable accuracy the true resistivity of the surrounding invaded formation and uninvaded formation, which insofar as the inventor is aware has never been heretofore possible.

In the apparatus illustrated in Figures 4 to 5 b, inclusive, when employing 144 pickup electrodes as beforementioned, it has been found that suitable speeds of operation of the commutator device may be between approximately l/16 to 1 revolution per second. It is desirable that in case of operation of the electrologging system with an alternating current of say 144 cycles per second, that at least one cycle and preferably two cycles, pass through the circuit during contact of the commutator device with any one of the commutator segments. At this frequency, the commutator is operated at either 30 or 60 revolutions per minute. In a case where, for example, a square wave alternating current of 15 cycles per second is employed, as disclosed in co-pending application Serial No. 17,478, now Patent No. 2,569,867, the commutator device may be operated at a speed of 6%. revolutions per minute in order to permit at least one full cycle to pass through the measuring circuits during contact of the brushes of the commutator device with any one of the commutator segments.

It is to be understood that the foregoing is illustrative only and that the invention is not limited thereby but may include various modifications and changes made by those skilled in the art without distinguishing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In an electrical well logging system wherein means including a current electrode and a potential detector respectively establish an electric field by current ow at a first point in a well bore and probe said electric field at a second point and wherein said electrode and detector are supported for movement along said well bore to vary the depth of said points for producing on a record element a plot of the electrical character of earth formations, the combination which comprises means for varying the spacing of said points between two different values, means including translating and registering means connected to said potential electrode for producing a scalar output having a magnitude dependent upon said electric eld and said spacing, means for adjusting the position of said registering means along a first dimension of said record element in proportion to the depth of said points in said well bore, and means for adjusting the position of said registering means along a second dimension of said record element in proportion to the value of said spacing between said points for registering said scalar output at a situs on said element dependent upon said depth and said spacing.

2. In an electrical well logging system wherein means including a current electrode and a potential electrode respectively establish an electric field by current ow at a first point in a well bore and probe said electric field at a second point and wherein said electrodes are supported for movement along said well bore to vary the location of said points for producing on a record element a plot of the electrical character of earth formations, the combination which comprises means for cyclically Varying the spacing between said points from a predetermined minimum to a predetermined maximum, means including translating means connected to said potential electrode for producing a scalar output which Varies in accordance with said electric field as said spacing and the position of said points are varied, means for varying in a first direction the positional relation between said translating means and said record element to correspond with variations in the position of said points in said well bore, and means for varying in a second direction the positional relation between said translating means and said record element to Correspond with variations in said spacing for registering said scalar output at a situs on said element dependent upon said position and said spacing. k

3. In an electrical well logging system wherein means including a current electrode and a potential detector, respectively, establish an electric iield by current ilow at a iirst point in a well bore and probe said electric tield at a second point and wherein said electrode and detector are supported for movement along said well bore to vary the depth of said points for producing on a record element a plot of the electrical character of earth formations, the combination which comprises: means for varying the spacing apart of said points between different values; means connected to said potential' electrode for producing a scalar output having a magnitude dependent upon the value of said electric iield; means for recording the magnitude of said scalar output along a first dimension of said record element in correlation with the depth of said points in said well bore and along a second dimension of said record element in correlation with the value of said spacing between said points, for recording the said magnitude of said scalar output at a situs on said record element dependent upon said depth and said spacing.

4. in an electrical well logging system wherein means including a current electrode and a pair of potential pickup electrodes respectively establish an electric field by current flow at a first point in a borehole and probe said electric field at a pair of other points therein and wherein said current electrode and said pair of potential pick-up electrodes are supported for movement along said bore hole to vary the depth of said points therein, for producing on a record element a plot of the electrical character of earth formations, the combination which comprises: means for varying the spacing apart of said first point with respect to said pair of other points between different values; means responsive to the potential different between said pair of other points for producing an output signal having a magnitude dependent upon the value of the electric field therebetween; means for recording the magnitude of said output signal along a rst dimension of said record element in correlation with the depth of said points in said borehole, and along a second dimension of said record element in correlation with the said value of said spacing between said first point and said pair of points, for recording the magnitude of said output signal at a situs on said record elem-ent dependent upon said depth and said spacing.

5. In an electrical well logging system apparatus comprising: an input electrode; means to flow an electric current between said input electrode and a remote electrode which is grounded; means to pick up potential differences resulting from said current and occurring between a pair of axially spaced-apart localities in the well borehole which are separated axially from said input electrode; means to vary the depth of said current electrode and said pick-up means in a well borehole; means to vary the effective axial separation between said input electrode and said pair of localities from which said potential differences are picked up; means responsive to the said potential differences picked up between said pair of spaced-apart localities for producing an output signal having magnitudes dependent upon the value of said potential differences; means for recording the magnitude of said output signal along a iirst dimension of a record element in correlation with the depth of said input electrode and said spaced-apart localities in said borehole; and along a second dimension of said record element in correlation with the said axial separation between said input electrode and said pair of localities.

6. In an electrical well logging system apparatus cornprising: an input electrode; means to flow an electric current between said input electrode and a remote electrode which is grounded; means to pick up potential differences resulting from said current and occurring between a pair of axially spaced-apart localities in the well borehole which are separated axially from said input electrode; means to vary the depth of said current electrede and said pick-up means in a well borehole; means to vary the effective axial separation between said input electrode and said pair of localities from which said potential differences are picked up; means to modify the thus picked-up potential differences substantially to eliminate differences thereof at different pick-up localities which would be solely the function of the said variation of the said effective axial separation; means responsive to the said potential differences picked up between said pair of spaced-apart localities for producing an output signal having magnitudes dependent upon the value of said potential differences; means for recording the magnitude of said output signal along a first dimension of a record element in correlation with the depth of said input electrode and said spaced-apart localities in said borehole; and along a second dimension of said record element in correlation with the said axial separation between said input electrode and said pair of localities.

References Cited in the tile of this patent UNITED STATES PATENTS 1,440,778 Foster Jan. 2, 1923 1,894,328 Schlumberger Ian. 17, 1933 2,105,247 Iakosky Ian. 11, 1938 2,117,390 Zuschlag May 17, 1938 2,133,786 Neutield Oct. 18, 1938 2,192,404 Jakosky Mar. 5, 1940 2,256,742 Iakosky Sept. 23, 1941 2,297,754 Ennis Oct. 6, 1942 2,313,384 Lee Mar. 9, 1943 2,317,259 Doll Apr. 20, 1943 2,317,304 Schlumberger Apr. 20, 1943 2,393,009 Shun Jan. 15, 1946 2,412,363 Silverman Dec. 10, 1946 2,632,795 Boucher Mar. 24, 1953

Claims

1. IN AN ELECTRICAL WELL LOGGING SYSTEM WHEREIN MEANS INCLUDING A CURRENT ELECTRODE AND A POTENTIAL DETECTOR RESPECTIVELY ESTABLISH AN ELECTRIC FIELD BY CURRENT FLOW AT A FIRST POINT IN A WELL BORE AND PROBE SAID ELECTRIC FIELD AT A SECOND POINT AND WHEREIN SAID ELECTRODE AND DETECTOR ARE SUPPORTED FOR MOVEMENT ALONG SAID WELL BORE TO VARY THE DEPT OF SAID POINTS FOR PRODUCING ON A RECORD ELEMENT A PLOT OF THE ELECTRICAL CHARACTER OF EARTH FORMATIONS, THE COMBINATION WHICH COMPRISES MEANS FOR VARYING THE SPACING OF SAID POINTS BETWEEN TWO DIFFERENT VALUES, MEANS INCLUDING TRANSLATING AND REGISTERING MEANS CONNECTED TO SAID POTENTIAL ELECTRODE FOR PRODUCING A SCALAR OUTPUT HAVING A MAGNITUDE DEPENDENT UPON SAID ELECTRIC FIELD AND SAID SPACING, MEANS FOR ADJUSTING THE POSITION OF SAID REGISTERING MEANS ALONG A FIRST DIMENSION OF SAID RECORD ELEMENT IN PROPORTION TO THE DEPTH OF SAID POINTS IN SAID WELL BORE, AND MEANS FOR ADJUSTING THE POSITION OF SAID REGISTERING MEANS ALONG A SECOND DIMENSION OF SAID RECORD ELEMENT IN PROPORTION TO THE VALUE OF SAID SPACING BETWEEN SAID POINTS FOR REGISTERING SAID SCALAR OUTPUT AT A SITUS ON SAID ELEMENT DEPENDENT UPON SAID DEPTH AND SAID SPACING.